The present invention relates to iron composite particles for purifying soil or ground water, a process for producing the iron composite particles, a purifying agent containing the iron composite particles, a process for producing the purifying agent, and a method for purifying soil or ground water. More particularly, the present invention relates to iron composite particles for decomposing and insolubilizing (1) organohalogen compounds, for example, aliphatic organohalogen compounds such as dichloromethane, carbon tetrachloride, 1,2-dichloroethane, 1,1-dichloroethane, cis-1,2-dichloroethylene, 1,1,1-trichloroethane, 1,1,2-trichloroethane, trichloroethylene, tetrachloroethylene, 1,3-dichloropropene or the like, aromatic organohalogen compounds such as dioxins, PCB or the like, (2) heavy metals such as cadmium, lead, chromium, arsenic, selenium, cyanogens or the like, which are contained in soil or ground water, in an efficient, continuous and economical manner; a process for producing the iron composite particle; a purifying agent containing the iron composite particles; a process for producing the purifying agent; and a method for purifying soil or ground water using the purifying agent.
The above aliphatic organohalogen compounds such as trichloroethylene, tetrachloroethylene or the like have been extensively used for cleaning in semiconductor-manufacturing factories and for degreasing metals to be machined.
Also, waste gases, fly ashes or main ashes discharged from incineration furnaces for combusting municipal garbage or industrial wastes, contain aromatic organohalogen compounds such as dioxins having an extremely high toxicity to human bodies even in a trace amount. The “dioxins” are a generic name of such compounds formed by replacing hydrogen atoms of dibenzo-p-dioxine, dibenzofuran, etc., with chlorine atoms. The waste gases or fly ashes continuously stay around the incineration furnace, so that the dioxins still remain in soil of surrounding regions.
Further, PCB (polychlorinated biphenyl) has been used in many application as insulating oils for transformers and capacitors, plasticizers or heating medium because of high chemical and thermal stability and excellent electrical insulating property thereof. Since the PCB is very harmful, the production and use thereof has been presently prohibited. However, any effective PCB-treating method has not been established until now and, therefore, a large part of the PCB past used has still been stored without treatment or disposal.
The organohalogen compounds such as aliphatic organohalogen compounds, aromatic organohalogen compounds or the like are hardly decomposable and besides exhibit carcinogenesis as well as a strong toxicity. Therefore, there arises such a significant environmental problem that soil or ground water is contaminated with these organohalogen compounds.
More specifically, upon discharge of the above organohalogen compounds, the hardly-decomposable organohalogen compounds are accumulated in soil, and the soil contaminated with the organohalogen compounds further causes contamination of ground water by the organohalogen compounds. In addition, the contaminated ground water flows out from the contaminated soil and spreads over the surrounding regions, so that the problem of pollution by the organohalogen compounds is caused over wider areas.
The soil is once contaminated with the organohalogen compounds, land involving the soil cannot be reused and developed again. Therefore, there have been proposed various techniques or methods of purifying the soil and ground water contaminated with the organohalogen compounds. However, since the organohalogen compounds are hardly decomposable and a large amount of soil and ground water must be purified, any efficient and economical purifying techniques or methods have not been fully established until now.
As the method of purifying soil contaminated with the organohalogen compounds, there are known a purifying method of using various catalysts; a method of absorbing and removing vapors of the organohalogen compounds by utilizing a volatility thereof; a thermal decomposition method of heat-treating excavated soil to convert the soil into harmless one; a method of purifying the soil by microorganisms; or the like. In addition, as to the ground water contaminated with the organohalogen compounds, there are known a method of extracting the contaminated ground water out of soil and converting the ground water into harmless one; a method of pumping the contaminated ground water and removing the organohalogen compounds therefrom; or the like.
Among these conventional methods of purifying soil or ground water contaminated with the organohalogen compounds, there have been proposed many methods of purifying the soil or ground water contaminated with the organohalogen compounds into harmless ones by mixing and contacting the soil or ground water with a purifying agent composed of iron-based particles (Japanese Patent Application Laid-Open (KOKAI) Nos. 11-235577(1999), 2000-5740, 2000-334063, 2001-38341, 2001-198567, 2002-161263, 2002-210452 and 2002-317202).
On the other hand, with recent increasing consciousness of environmental protection, the contamination of soil or ground water by heavy metals or the like has been noticed. In particular, soil or ground water contaminated by harmful substances including heavy metals such as cadmium, lead, chromium, arsenic, selenium, cyanogen or the like exerts significant influences on human bodies and ecosystem. Therefore, the development of methods for purification and removal of these harmful substances has also been urgently demanded.
As well known in the art, technical measures for treatment of soil or ground water contaminated with harmful substances such as heavy metals are classified into two categories, i.e., “purification techniques” and “containment”. Further, the purification techniques are classified into “in-situ purification” and “removal by excavation” in which contaminated soil is excavated and removed from objective lands. Furthermore, the “in-situ purification” techniques are classified into “in-situ decomposition” in which heavy metals or the like contained in the contaminated soil or ground water, are decomposed under the ground (in situ), and “in-situ extraction” in which the contaminated soil or ground water is extracted or excavated, and then heavy metals or the like contained in the soil or ground water are removed therefrom.
Further, the “in-situ extraction” techniques are classified into “decomposition” in which among objective substances belonging to the “heavy metals or the like”, compounds such as cyanogen and agricultural chemicals are thermochemically decomposed, and “separation” in which concentrated heavy metals, etc., are physically separated from the contaminated soil or ground water.
On the other hand, the “containment” techniques are classified into “in-situ containment” and “containment after removal by excavation”. The in-situ containment techniques are techniques of solidifying contaminated soil by mixing a solidifying agent therewith, and then confining the contaminated soil in situ without displacement therefrom. The techniques of containment after removal by excavation are techniques of pre-mixing an insolubilizing agent with contaminated soil to convert the soil into hardly-soluble one, drilling the contaminated soil once, and then confining the contaminated soil in place.
As the working methods for executing the “purification techniques”, there may be used a soil-washing method, a heat-desorption method or the like. For example, there may be used a chemical dissolution method in which chemicals are added to the contaminated soil to dissolve heavy metals or the like therein, and then the resultant solution is separated therefrom; a water-washing method of washing the contaminated soil with water and then classifying the soil to separate fine particles containing a large amount of heavy metals or the like therefrom; a soil wet-washing method of washing out contaminants adhered onto the surface of soil particles with a washing agent, and further classifying the soil particles into clean large particles and fine contaminant particles according to particle size and specific gravity thereof; or the like.
Also, in the “containment” techniques, as the working method for the “in-situ containment”, there is known a method of mixing a solidifying agent such as cement with the contaminated soil and then confining the solidified soil by a water-impermeable layer, steel sheet pile, etc. As the working method for the “containment after removal by excavation”, there is known a method of adding chemicals to the contaminated soil to insolubilize the soil and change the soil into hardly-elutable form, and then confining the hardly-elutable soil by insulating method or water-shielding method.
However, the above conventional treatment techniques undergo high treating costs, and require a prolonged treating time. Therefore, these techniques may fail to reduce harmful substances such as heavy metals or the like in an efficient and continuous manner.
In recent years, there have been developed low-cost treating techniques of reducing a valence of the heavy metals mainly on the basis of a reducing activity of iron particles in order to convert the heavy metals into harmless and stabilized form. For example, in Japanese Patent Application Laid-Open (KOKAI) No. 2001-198567, there is described the method of utilizing a reducing activity of iron particles to chromium (reduction in metal valence). In addition, in Japanese Patent Publication (KOKOKU) No. 52-45665(1977), it is described that when iron particles are added to a heavy metal ion-containing solution whose pH value is adjusted to about 5 to 6, and then the resultant mixture is stirred, a part of the iron particles are dissolved and precipitated in the form of ferric hydroxide which is then transformed into goethite or lepidocrocite with increase of the pH value, whereupon a part of the heavy metals are co-precipitated together with the goethite or lepidocrocite, so that a large part of the heavy metals are adsorbed into the resultant iron particles. Also, it is described that the amount of the iron particles eluted is increased at a low pH value, resulting in deteriorated adsorption/removal effect thereof.
In Japanese Patent Application Laid-Open (KOKAI) No. 11-235577(1999), there is described a method of adding and mixing in soil, iron particles containing carbon in an amount of not less than 0.1% by weight to convert organohalogen compounds contained in the soil into harmless ones. In this method, although the specific surface area and particle size of the iron particles used are specified, since the particle size is too large, it may be difficult to fully decompose the organohalogen compounds.
In Japanese Patent Application Laid-Open (KOKAI) No. 2000-5740, there is described the method of converting organohalogen compounds contained in soil into harmless ones using copper-containing iron particles. However, since decomposition of the organohalogen compounds requires a long period of time, this method may also fail to efficiently convert the organohalogen compounds into harmless ones.
In Japanese Patent Application Laid-Open (KOKAI) No. 2000-334063, there is described the method of contacting dioxins with an aqueous hydrochloric acid solution containing mill scale produced from the production process of hot-rolled steel plate in ironworks, at a temperature lower than 100° C. to convert the dioxins into harmless ones. However, since the use of the aqueous hydrochloric acid solution is essentially required in order to promote conversion of the organohalogen compounds into harmless ones, the decomposition reaction of the mill scale-by itself may fail to proceed sufficiently.
In Japanese Patent Application Laid-Open (KOKAI) No. 2001-38341, there is described a soil-purifying agent composed of a water suspension containing iron particles having an average particle diameter of 1 to 500 μm. However, since the iron particles used have a too large particle size, it may be difficult to fully decompose the organohalogen compounds.
In Japanese Patent Application Laid-Open (KOKAI) No. 2001-198567, there is described the method of using a water suspension containing spherical iron particles having an average particle diameter of less than 10 μm. Since the water suspension containing the spherical iron particles is obtained by collecting dusts contained in waste gas discharged during refining process from an oxygen blowing converter for steel-making, and removing gases from the dusts, it may be difficult to fully reduce the organohalogen compounds.
Further, the above method described in Japanese Patent Application Laid-Open (KOKAI) No. 2001-198567 is the method of converting the harmful substances into harmless and stabilized ones by utilizing the reducing activity of the iron particles (reduction in valence). However, the iron particles suffer from deterioration in the reducing activity with the elapse of years, thereby failing to continuously maintain the reducing activity. As a result, even though the heavy metals are temporarily converted into those having a stable and harmless low valence, there is a possibility that the valence is thereof increased again, so that the heavy metals are converted into previous harmless ones. Therefore, the above method fails to provide a long-term effective purification method.
In Japanese Patent Application Laid-Open (KOKAI) No. 2002-161263, there are described iron particles for decomposing organohalogen compounds in which a part of the surface of the iron particles is adhered with a metal selected from nickel, copper, cobalt and molybdenum, and the remaining part of the surface other than the surface adhered with the above metal is covered with an iron oxide film. However, the iron particles used are iron particles obtained from mill scale or iron particles obtained by atomizing molten steel with water. As is apparent from the specific surface area of the iron particles as described therein, it is considered that the iron particles have a large particle size. Thus, the iron particles may also fail to fully reduce the organohalogen compounds.
In Japanese Patent Application Laid-Open (KOKAI) No. 2002-210452, it is described to use sulfur-containing iron particles for purification treatment of soil or ground water contaminated with organohalogen compounds. However, since the iron particles have a too large particle size, it may be difficult to fully reduce the organohalogen compounds.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 2002-317202, it is described to use magnetite-containing iron composite particles for purification treatment of soil or ground water contaminated with organohalogen compounds. However, since the iron particles contain no sulfur, it may be difficult to fully reduce the organohalogen compounds.
In addition, the method described in Japanese Patent Publication (KOKOKU) No. 52-45665(1977) utilizes mainly the reducing activity or absorption activity of the iron particles. Although these activities are attained by a partial dissolution of the iron particles, the method is based on such a mechanism that when the iron particles are eluted over a whole acidic range and then converted into goethite, lepidocrocite or magnetite, the heavy metals are included therein. Therefore, this method is not a method of positively using such a phenomenon that the iron particles are dissolved in the form of Fe2+ or Fe3+ and formed into spinel ferrite while incorporating heavy metals therein.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that resultant iron composite particles obtained by cooling iron particles obtained by heat-reducing goethite or hematite particles having a specific average particle diameter and a specific Al content at a temperature of 350 to 600° C.; transferring the obtained iron particles into water without forming a surface oxidation film on surface of the iron particles in a gas phase; forming the surface oxidation film on the surface of the iron particles in water; and then drying the iron particles provided with the surface oxidation film, are capable of treating organohalogen compounds and/or heavy metals contained in the soil or ground water in an efficient, continuous and economical manner. The present invention has been attained on the basis of this finding.